Such a planar antenna assembly usually comprises i) a ground plane and a feeding circuit defined on a face of a printed circuit board, ii) feed and shorting tabs coupled to the feeding circuit and the ground plane respectively, and iii) a radiating element connected to the feed and shorting tabs and in which a slot (comprising opened and closed ends) is defined in a plane parallel to the ground plane. An example of such a planar antenna assembly is notably disclosed in patent document EP 1502322.
This kind of antenna assembly is advantageous not only because of its limited bulkiness but also because it may allow multi frequency working (and multi-standard working) when it is connected to a switching circuit. Unfortunately, in this kind of antenna assembly the input impedance varies with the operating frequency. Therefore it becomes difficult to match the antenna assembly to the commonly used 50 ohms impedance of the RF communication equipment or module over a wide frequency range or large number of frequency bands. Moreover, in equipment such as mobile phones, the slot is located in a plane parallel to the front and back covers (defining the casing) in an area where the user's hand interacts with it, causing detuning and degradation of the radio performance.